Technical Field
The present disclosure relates to a pinned photodiode with a low dark current, capable of forming a pixel of an array image sensor.
Description of the Related Art
Accompanying FIG. 1 corresponds to FIG. 2 of U.S. Pat. No. 6,677,656. This drawing is a partial simplified cross-section view of a monolithic embodiment of the assembly of a photodiode and of an associated transfer transistor. These elements are formed in a same active area of an epitaxial layer 3 formed on a semiconductor substrate 1. The active area is delimited by field insulation areas 2, for example, made of silicon oxide (SiO2). Epitaxial layer 3 is lightly P-type doped. Substrate 1 is also of type P but more heavily-doped. Above the surface of layer 3 is formed an insulated gate structure 4, possibly provided with lateral spacers. On either side of gate 4, at the surface of well 3, are located N-type source and drain regions 5 and 6. Drain region 6, to the right of gate 4, is heavily doped (N+). Source region 5 is formed on a much larger surface area than drain region 6 and forms the useful region where photons are converted into electric charges. Gate 4 and drain 6 are integrate with metallizations (not shown). The structure is completed with heavily-doped P-type regions 8 and 9 (P+). Regions 8 and 9, bordering insulating areas 2, are connected to the reference voltage or ground via well 3 and substrate 1.
The photodiode further comprises, at the surface of its source region 5, a P-type layer 7 in lateral contact with region 8. It is thus permanently maintained at the reference voltage level. The useful region where photons are converted into electric charges, source region 5, is electrically floating. Such a photodiode is called “pinned diode”.
FIG. 2 shows the distribution of the dopant atom concentrations in a direction x perpendicular to the main plane of layers 7, 5, 9, and 3. In the shown case, epitaxial layer 3 is of constant doping, for example, in the range from 5×1014 to 3.1015 at./cm3. The N region 5 is obtained by implantation and has a maximum concentration in the range from 1016 to 8×1017 at./cm3. Layer 7 is obtained by implantation. Inevitably, if the implantation dose used to form layer 7 is higher, the junction depth between regions 7 and 5 is larger. The case of a first maximum concentration c1 in the range from 1 to 2×1018 at./cm3 for which the junction depth is equal to xj1 in the range from 30 to 50 nm and the case of a second implantation having a maximum concentration in the range from 5×1018 to 5.1019 at./cm3 and having a junction depth equal to xj2 in the range from 100 to 250 nm have been shown.
If the photons, and particularly the photons corresponding to blue, are desired to be properly absorbed, layer 7 should be as thin as possible. Indeed, in blue (for a 450-nm wavelength), substantially 50% of the photons are absorbed in the first 170 nm. The thickness of layer 7 should thus be much smaller than this value. As a result, its maximum concentration, and particularly its surface concentration, is no greater than 1018 at./cm3. In a practical implementation, after having formed the structure of FIG. 1, said structure is coated with an insulator layer, currently silicon oxide. Now, it is known that at the interface between a doped region and silicon oxide, a temperature-activated generation of electron-hole pairs, some of which will go into the N layer, will occur. Thus, even in the absence of illumination, region N charges. This corresponds to what is called dark current. This dark current is desired to be as low as possible.